1. Field of the Invention
The present invention relates to an implantable heart stimulating device with which it is possible to stimulate both the ventricles of a heart, i.e. a bi-ventricular pacer.
The invention also relates to a system including such a device and to a method for his bi-ventricular stimulation and sensing.
2. Description of the Prior Art
Many different implantable devices for stimulating a heart are known. The devices are normally able to sense the electrical activity of the heart. Some implantable devices are able to deliver stimulation pulses to both the left and right ventricles of the heart, and sometimes also to the left and right atria.
Devices that are able to deliver stimulation pulses to both the left and right ventricles are also called bi-ventricular pacers. Such devices can be used to treat patients who suffer from different severe cardiac problems, e.g. patients suffering from congestive heart failure (CHF). CHF is defined generally as the inability of the heart to deliver a sufficient amount of blood to the body. CHF can have different causes. It can for example be caused by a left bundle branch block (LBBB) or a right bundle branch block (RBBB). By using bi-ventricular pacing, for example, the contraction of the ventricles can be controlled in order to improve the ability of the heart to pump blood. The stimulation pulses to the two ventricles can be delivered simultaneously but it is also known for the stimulation pulses to the two ventricles to be delivered with a short time delay between them in order to optimise the pumping performance of the heart.
U.S. Pat. No. 5,720,768 describes different possible electrode positions in order to stimulate or sense the different chambers of the heart.
U.S. Pat. No. 6,070,100 describes that electrodes may be positioned in both the left and the right atrium as well as in the left and the right ventricles.
In connection with implantable pacers, in particular pacers which only have the possibility to stimulate the right ventricle, and sometimes also the right atrium, it is known to detect capture, i.e. to detect whether the heart actually reacts to a delivered stimulation pulse. If capture is not detected it is possible to cause the pacer to deliver a back-up pulse with a higher pulse energy than the first pulse. It is also possible to increase the pulse energy in future stimulation pulses if capture is not detected. In order to conserve the battery it is important for stimulation pulses not to be delivered with an unnecessarily high energy. By varying the energy of the stimulation pulses and by detecting the capture it is possible to find a threshold value for the stimulation pulse energy. Based on the threshold value, a suitable stimulation pulse energy can be determined.
The detection of capture involves several problems. Different signals from the heart or generated by the pacemaker may interfere with each other, which may make the detection of capture difficult. The evoked response that it is intended to detect may thus be hidden because of other electrical phenomena. It is particularly difficult to detect capture in a bi-ventricular pacer since in such a pacer there are more delivered and detected signals which may interfere with each other.
U.S. Pat. No. 6,148,234 describes a system for detecting capture in connection with bi-ventricular or bi-atrial pacing. The document describes the fact that if a chamber is captured, then there is a biological refractory period during which this chamber cannot be stimulated again. The system described in this document monitors these refractory periods for the different chambers, for example for the two ventricles. When capture is achieved in both ventricles, no intrinsic depolarization signals can be sensed during the following refractory period. However, where the output level of one of the pacing pulses is insufficient to capture one ventricle, but capture is achieved in the other ventricle, a delayed depolarization pattern can be detected in the ventricle that was not captured. This delayed depolarisation is due to an interventricular conduction from the ventricle that is captured to the ventricle that is not captured. The system according to this document thus monitors the refractory interval following each delivery of stimulating pulses to the ventricles. A loss of capture is indicated in case such a delayed depolarisation is sensed during the refractory period.
Also United States Patent Application Publication 2001/0049542 describes a system for detecting capture in connection with bi-ventricular or bi-atrial stimulation. The system includes a morphology detector incorporated in a micro controller to allow for the processing of the sensed intra-cardiac electrogram signals (IEGM). The morphology of the IEGM may depend on whether both the ventricles (or atria) have captured or not. By detecting the shape of the IEGM capture thus may be detected.